1. Field of the Invention
This invention relates to a time division digital transmission system in which data are transmitted in the form of pulses grouped in multiplex words in time, in time slots forming a recurrent frame; the invention relates more particularly to facilities in transmission-line stations for sampling from and introducing into the word frame data words taking up particular time slots in the latter frame.
2. Description of the Prior Art
In known systems of the kind described, component digital signals which are either synchronous or which have been made synchronous by stuffing, e.g. signals at a rate of 64 kbits/sec, are time multiplexed in a transmitting station for transmission over a high-rate multiplex digital channel whose rate is e.g. 2.048 Mbits/sec if the multiplex digital channel arise from the multiplexing of 32 64 kbits/sec digital signals. The component digital signals occupy the time slots in the frame of the resulting digital signal and the origin of each frame is marked by an individual recognisable word called the framing word. The frames are demultiplexed at a receiving station to restore the component digital signals. At intermediate stations on the high-rate multiplex digital channel some component digital signals can be sampled from the incoming frame by demultiplexing or inserted in the outgoing frame by multiplexing.
Frame demultiplexing at terminal and intermediate stations is carried out by a reception time base which is synchronised by the incoming signals. The reception time base is a circuit which restores from the incoming frame the e.g. 3.9 .mu.s pulses representing the word time slots and the e.g. 250 ns pulses separated by 250 ns intervals and representing the clock bits, the reception time base supplying the addresses from 0 to 31 for the time slots and from 1 to 8 for the clock bits, as is conventional for TN 1 multiplex channels.
3. Control Problem of the Prior Art
The reception time base is controlled by clock pulses taken from the incoming frame and is frame-synchronised by a framing circuit and ceases to operate when there are no incoming signals. Consequently, in the event of a breakdown upstream of an intermediate or terminal receiving station either at the actual transmission stations or somewhere along the chain of transmission, the reception time base of the receiving station concerned loses frame sync and stops. Synchronism must therefore be restored when normal signal transmission will take place again. If the receiving station is an intermediate station, this results in an interruption of the digital signal downstream of the receiving station.